Optical fiber sensors capable of simultaneously measuring distributed strain and temperature have generated significant interest in the technical community. Current technologies have two limitations. They are either difficult to manufacture because they require multi-step manufacturing procedures or they have limited multiplexing potential. For example, the cascaded Bragg grating/extrinsic Fabry-Perot and intrinsic Fabry-Perot/extrinsic Fabry-Perot have excellent numerical stability, have proven very effective in measuring strain and temperature in combined thermomechanical fields, and satisfy the condition of having the same or similar gage lengths for the strain and temperature components of the sensor. However, the extrinsic Fabry-Perot component of the sensor is not always easy to manufacture, and has limited multiplexing potential (maximum of approximately 5 sensors). The other sensor configuration that is promising is the superposed grating configuration proposed by Xu et al., Electronic Letters, 30 (13), pp. 1085-1087, 1994. This technology does not enable cost effective manufacturing. Superposing two gratings is simply not compatible with present day Bragg grating mass production facilities available at Bragg grating manufacturers.
The concept of a two-Bragg condition grating sensor was recently proposed by Brady et al., IEEE Proc.--Optoelectrons, Vol. 144, No. 3, June 1997, pages 156-161. However, Brady et al. did not attempt to make simultaneous measurements of strain and temperature. They were only able to achieve .about.0.1 percent reflectivities of second order Bragg condition. The small reflectivity of the second order Bragg condition of the grating necessitated the use of overly complex method of monitoring the two Bragg wavelengths.